A variety of applications use polyphase direct current (dc) motors for providing rotational motion. In particular, applications such as hard disk drives and CD-ROM drives often use polyphase dc motors, such as three-phase dc motors, to rotate information platters including the magnetic disks of a hard disk drive. The control of the rotational speed of these information platters is often critical to overall application performance.
The rotational speed of these polyphase dc motors is controlled through the current applied to the stator windings or coils. For example, the stator windings of a three-phase dc motor may be coupled in a "Y" configuration and include an A-coil, a B-coil, and a C-coil coupled at one end at a center tap node. The remaining ends of each coil are selectively coupled to either a high side driver, a low side driver, or to an open circuit as commutation occurs. During steady state operation, current flows from a high side driver, through a first coil coupled to the high side driver, through the center tap, through a second coil coupled to the low side driver, and to the low side driver. During this time, a third coil couples to the center tap on one end while the other end is provided as an open circuit. After a period of time, a commutation occurs so that current may now flow through the third coil and either the first coil or the second coil. A commutation is the transfer of current from one path in a circuit to another. Thus, current flows through two of the three coils during a steady state operation until a commutation occurs, at which time, current then flows through one of the two coils and the third coil until the next commutation occurs.
A total of six currents may be provided in the stator windings of a three-phase dc motor through six commutations. The current flows, for example, may be provided through the stator coils in the following sequence to impart rotational motion to the rotor of the three-phase dc motor: A-coil to C-coil, A-coil to B-coil, C-coil to B-coil, C-coil to A-coil, B-coil to A-coil, and B-coil to C-coil.
Problems arise when commutating the current in the coils of the stator winding. Often, current and voltage spikes occur as a result of a commutation. These current and voltage spikes occur when current is reduced in one coil and increased in another. These current and voltage spikes may damage sensitive control circuitry. For example, voltage sensitive circuitry such as metal oxide semiconductor ("MOS") circuitry is often destroyed if breakdown voltages are violated. These current and voltage spikes require the use of integrated circuit technology and transistor technology in the control circuitry having breakdown voltages greater than the voltage spikes generated during a commutation. This results in larger geometry control circuitry translating into increased circuit size and increased power consumption.
Furthermore, additional circuitry, such as an external zener diode, must be included in the control circuitry to prevent the control circuitry from being damaged in the event that a current or voltage spike causes a breakdown voltage level to be exceeded. This further increases overall control circuitry costs, complexity, and power consumption. Also, overall system reliability suffers because in the event of failure of the external zener diode, the control circuitry may be destroyed.
Additional problems are caused by the presence of current and voltage spikes generated during commutations in the stator windings. Current and voltage spikes may create a torque ripple and establish a resonant frequency resulting in an audible noise in the 2-4 kHz range, which is a typical commutation frequency. The torque ripple may harm overall hard disk drive performance by introducing data errors caused by attempting to read data during a torque ripple.